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Old 6th December 2007, 11:21 PM   #1
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Join Date: Nov 2007
Default Please review my guitar amp plan (high level)


I have decent (maybe) theoretical understanding of electronics from high school etc. I have never put it to practical use other then building a couple of simple kits. I have reasonable wood working skills and access to most tools.

My 13 year old son has been playing guitar for over a year now and has demonstrated that he is keen to continue. I would like to upgrade his amp for him. He is currently using the Squire SP-10 that came with his guitar.

I have done a bit of reading (including the "Solid State Guitar amps worthwhile??" thread and a lot of stuff referenced from it) and it seems that amps are a very personal choice. He does not yet have enough experience to make a very informed choice :-) He will also almost certainly want to upgrade his guitar soon as well so I don't want to spend a lot on an amp.

Here is my plan...

- Build a head and 2x12 box. 2 pieces for easier carrying. 2x12 because I have some old 12" drivers that should be OK for starters.

- Build 2 x LM1875 power amps using the schematic from the data sheet.

- Build a TL082 based pre-amp using the schematic from the data sheet.

- Build 1 or 2 effects from to put in front of the pre-amp with appropriate switch capability - 1, both or neither in the path.

My rationale is that this is modular and can be incrementally improved as he "grows". It is also not a huge investment so I can abandon it if the whole thing is a flop. I'd love to build him something tube based but with his lack of experience (to know what he wants) and my lack of knowledge/experience it is just not an option. Maybe next time...

I'll need some help from "you" though...

1. Does this sound like a reasonable plan?

2. Is the the schematic on the TL082 data sheet even a pre-amp?? :-)

3. I know that most of the circuits on are intended for use in stomp boxes. Can you suggest some that might be good general "basic" effects that could be easily modified to wire into the cabinet? I am concerned about matching sensitivity between the various devices, but as they seem to be designed to chain together I'm not sure if I need to be - does it depend on the ones I choose? Should I just build a simple, "clean" amp and keep the stomp boxes external?? (I think he would want at least 1 distortion built-in...)

I've got a few more questions, but they are not worth asking yet...


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Old 6th December 2007, 11:32 PM   #2
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Sounds OK. Ditch the TL072 pre and use the ROG Thor and/or Mtchbox as a pre. Be prepared to experiment with JFETs to get desired tone and gain.
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Old 7th December 2007, 01:41 AM   #3
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OK, but is that going to give a clean channel to run other effects through? Or, do I just need to make sure that any other effects I build (or he gets elsewhere) have enough gain to drive the power amp? Will this restrict him sharing/swapping stuff with his friends, or do most/all effects have enough gain?


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Old 7th December 2007, 02:24 AM   #4
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It's fairly clean or at least the one I have breadboarded around here is, but that depends on the JFETs selected. It'll have enough gain to drive a poweramp. Most FX pedals don't have any gain.
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Old 11th December 2007, 05:42 AM   #5
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Hi friend,
Using the tonemender project from
as your preamp could be much better than the TL082
datasheet project. I'm using it this way with very good
results, worth to try.

If you want a volume control that can make it zero volume
just use a 120K resistor (25x aprox.) in the place of level
pot and use a 100K pot in the output. My tests give full
output from a LM3886 amp using this configuration but
I think you can set it with even more gain. Just an idea
for a really flexible and simple preamp.

Hope this info help in some way, sorry about my poor
english. Thanks a lot and good luck.

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Old 11th December 2007, 07:18 AM   #6
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Join Date: Nov 2007
I had seen that one and thought about it. But, I am probably going to use the ROG Thor as the pre. I have ordered all the bits to make it as an effects box. If my son likes the sound he can get with Thor through the clean channel of his current amp I will take it out of the effects box and use it as the pre amp. If not, I'll make another circuit for the box and keep trying...

I've also changed the rest of the "design" as well :-) I've been reading a fair bit more and am thinking that I will probably use a single LM3886 rather than 2 LM1875s for the power. I was thinking 1 amp per speaker, but I have realised that the current drivers I have will be no good for anything other than proof-of-concept for the look of the box. I am going to need to buy a real driver so it will probably just be a 1 x 12. Now that it is only 1 x 12 a combo will be manageable for him to lug around...

I still need to experiment with the sound of the Thor once I get it built as well - he is keen to have a nice clean channel so I might need to build the vanilla TL082, or maybe the Tonemender, as well.


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Old 11th December 2007, 08:59 AM   #7
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Location: Santos Lugares
You can use the SamsAmp GT2 for the pre. It emulates the Fender Tweed, Marshall and Mesa with different configurations from clean to hot wired. Ive used it in a small practice amp that I use. Is not an easy project but sounds great ! You can see the information here :

Hope that helps.
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Old 11th December 2007, 12:47 PM   #8
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Ok friend,
Just take care to not use a hi-fi style preamp
or amp for guitar, at beginning it sounds good,
clean and musical, but along a band you will feel
that something is really wrong. For your LM3886
amp I suggest the project below:
But, add a feedback resistor, works fine.


About the GT2 pre... Can it handle another distortion
before it? at the clean configuration? I'm in process
os construction (pcb finished) but don't know too much
about the circuit. If it can, is a really good pre option too.
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Old 11th December 2007, 01:03 PM   #9
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Output impedance has little to do with it. OPT saturation is a far greater contribution to sonics than simple O/P Z.
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Old 11th December 2007, 01:48 PM   #10
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Don't think an OPT is necessary with a LM3886
project like this, sure a bomb will kill a fly, but...
2 resistor can do a good job too:
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